1. Field of the Invention
This invention relates to an optical disc, an optical disc reproducing apparatus and recording and reproducing methods for an optical disc.
2. Description of Prior Art
There has been known an optical disc on which music information or picture information is recorded, and an optical disc reproducing apparatus for reproducing the recorded information from the optical disc. In such optical disc and optical disc reproducing apparatus, as shown in FIG. 1, the music information or picture information is recorded on an information recording surface 95 of the optical disc 92 in such a manner that information is converted to signals corresponding to an information pit 92 whose pit length serves as a parameter. In order to reproduce the information recorded on the optical disc 92 as pits, a laser beam is irradiated onto the information pit 91 in the form of a beam spot LS having a predetermined diameter, and a diffracted and reflected light from the information pit 91 is detected by a photodetector which includes a photodiode or the like to convert the detected light to an electrical signal. Music information or picture information recorded on the optical disc 92 is extracted from the electrical signal by a signal conversion processing converse to that of the recording operation, and output from the reproducing device. A compact disc (CD) and a laser video disc (LVD) are well known as the example of the optical disc 92 and the optical disc reproducing device as described above, respectively. In FIG. 1, a reference numeral 93 represents a substrate formed of polycarbonate resin or the like, and a reference numeral 94 represents a protection layer.
As seen from FIG. 2, the information recording density of the optical disc is greatly dependent on both of a track pitch P1 which is defined as an interval between the central lines of bit arrays on which information pits 91 are formed, and the diameter of the beam spot LS of the laser beam which is irradiated onto the information pits 91. In view of this, various attempts for making the track pitch of an optical disc narrower than the current track pitch are made so as to increase the amount of information recorded on the optical disc.
However, the following problem occurs in a case as shown in FIG. 3 where the track pitch is set to a value P2 which is smaller than a currently-adopted value P1 (for example, P2=P1/2). That is, as shown in FIG. 3, in addition to a target information pit 91A to be read out by a beam spot LS, neighboring information pits 91B and 91C locating at both sides of the track of the information pit 91A are unfavorably read out by the beam spot LS. Consequently, since a cross-talk amount is increased, such an optical disc having narrower track pitch can not be practically used.
On the other hand, another attempt to reduce the diameter of the beam spot LS of the laser light has been made. The beam diameter w when a laser beam having wavelength .lambda. is focused to a position of focus distance f by an objective lens OL as shown in FIG. 4 is represented by the following equation. EQU w=1.22.times..lambda./NA (1)
Here, NA represents the numerical aperture of the objective lens OL. When the refractive index of the lens is n and the angle of emergence of the laser light with respect to the lens is .theta., NA is represented by the following equation. EQU NA=N.times.sin.theta. (2)
Therefore, the laser beam diameter w can be reduced by shortening the wavelength .lambda. of the laser light or increasing the numerical aperture NA. Regarding the wavelength .lambda. of the laser light, the wavelength of a semiconductor laser which is presently used for an optical disc is about .lambda.=0.780 .mu.m (.mu.m=10.sup.-6 meter). Regarding the numerical aperture NA, NA is approximately 0.45 for a CD. Therefore, the minimum laser beam diameter w.sub.min is approximately equal to 2.1 .mu.m by the following equation. EQU w.sub.min =1.22.times.0.780/0.45=2.1 (.mu.m)
Accordingly, the minimum track pitch which provides no crosstalk when the laser beam spot is irradiated onto a bit array is about 1.6 .mu.m, and this value is adopted in many optical discs which have been practically used at present.